A self-emulsification-type aqueous polyurethane internally having a hydrophilic group is broadly applied to paints, adhesives, or fluid dispersions used for treating fibers.
As such an aqueous polyurethane having a hydrophilic group, aqueous polyurethanes having a carboxylate derived from dimethylol propionic acid (DMPA), dimethylol butanoic acid (DMBA), or the like, are known. However, the aqueous polyurethane having a carboxylate lacks in: storage stability due to its readily hydrolyzable properties; stability against pH; and thermal resistance of coating film. In addition, carboxylate is inferior to sulfonate in terms of hydrophilicity, and therefore the carboxylate is required to be introduced in the polyurethane at a high concentration so as to obtain a stable polyurethane fluid dispersion.
On the other hand, sulfonate is conventionally introduced by reacting N-(2-aminoethyl)-2-aminoethane sulfonate (AAS salt) (see, for example, Patent Document 1) with a prepolymer having an isocyanate at an end thereof, or by introducing sulfosodium isophthalate into polyol to obtain polyurethane (see, for example, Patent Document 2). Since the sulfonate-based polyurethane has a higher hydrophilicity than the carboxylate-based polyurethane, it is considered that the content of hydrophilic group in the sulfonate-based polyurethane can be decreased and deterioration of hydrolysis resistance or thermal resistance can be suppressed. Accordingly, the sulfonate-based aqueous polyurethane is preferably applied to adhesives, in particular.
However, the structure of the conventionally known hydrophilic group destabilizes the dispersion if the conventionally known hydrophilic group is at a certain concentration or less, and therefore there is a limitation on the improvement of the properties by decreasing the concentration of hydrophilic group, although the concentration of hydrophilic group in polyurethane is required to be low so as to further improve properties.    [Patent Document 1] Canadian Patent No. 928323, Description    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H10-110021.